1. Field of the Invention
The invention relates to an image forming apparatus using an electrophotographic printing method, an electrostatic recording method or the like, and particularly to an image forming apparatus which collects a toner residual on an image bearing member after the transferring step by developing means and reuses it.
2. Related Background Art
An image forming apparatus such as a copying machine, a printer or a facsimile apparatus using the electrophotographic printing method generally has a cylindrical electrophotographic photosensitive member (photosensitive member) as an image bearing member. Also, the image forming apparatus has a charging device (charging step) for uniformly charging the photosensitive member to a predetermined polarity and potential, and an exposing device (exposing step) as information writing means for forming an electrostatic image on the charged photosensitive member. Also, the image forming apparatus has a developing device (developing step) for visualizing the electrostatic image formed on the photosensitive member as a developer image (toner image) with a toner as a developer, and a transfer device (transferring step) for transferring the toner image from the surface of the photosensitive member to a transfer material. Also, the image forming apparatus has a fixing device (fixing step) for finally fixing the toner image transferred to the transfer material such as recording paper. Further, generally, the image forming apparatus has a cleaning device (cleaning step) for removing any toner (residual developer or untransferred toner) more or less residual on the photosensitive member after the transferring step and cleaning the surface of the photosensitive member. As described above, in the image forming apparatus of the electrophotographic printing type, the photosensitive member is repetitively subjected to an electrophotographic process (the charging step, the exposing step, the developing step, the transferring step and the cleaning step) and is used for image formation.
The untransferred toner is removed from the surface of the photosensitive member by the cleaning device, is collected into the cleaning device and becomes a waste toner. However, from such viewpoints as the preservation of the environment and the effective utilization of resources, it is desirable that such a waste toner does not come out.
From such viewpoints, there has been proposed an image forming apparatus, which returns the untransferred toner, collected in the cleaning device to the developing device and reuses it.
Also, there has been proposed an image forming apparatus of a “cleanerless type” in which the cleaning device is disused and the untransferred toner is removed and collected from on the photosensitive member by “cleaning simultaneous with developing” in the developing device, and is reused.
The cleaning simultaneous with developing collects the untransferred toner on the photosensitive member after the transferring step into the developing device during the developing step after the next step. That is, the photosensitive member to which the untransferred toner adheres is continuedly charged and exposed to thereby form an electrostatic image. Then, during the developing step for this electrostatic image, of the untransferred toner residual on the surface of the photosensitive member, the untransferred toner present on a portion (non-image portion), which should not be developed, is removed and collected into the developing device by fog removal bias. The fog removal bias is the potential difference (fog removal potential difference Vback) between a DC voltage applied to the developing device and the surface potential of the photosensitive member.
Here, the untransferred toner is not constant in the charging amount thereof and therefore, is difficult to intactly collect into the developing device. Therefore, there is a method of applying a bias to a charge assisting member or the like provided downstream of a transfer portion (transfer position) with respect to the rotation direction of the photosensitive member, to thereby uniformize the charging amount of the untransferred toner to a desired charging amount, and collect the untransferred toner by the developing device (Japanese Patent Application Laid-open No. 2004-117960).
According to such a cleanerless type, the untransferred toner is collected by the developing device and is reused for the development of the electrostatic image after the next step. Therefore, the waste toner can be eliminated, and cumbersomeness can be reduced during maintenance. Also, a cleaning member and a waste toner conveying mechanism are unnecessary, and this is also advantageous to the downsizing of the image forming apparatus.
There has also been proposed an image forming apparatus provided with a plurality of image forming portions of such a cleanerless type as described above (Japanese Patent Application Laid-open No. 2004-021178). That is, a plurality of image forming portions each adopting the cleanerless type are disposed along the movement direction (progress direction) of a transfer material. Yellow, magenta, cyan and black toner images are formed by the respective image forming portions, and are successively superposed and transferred to thereby obtain a full-color image formed article (a copy or a print).
However, it has been found that the image forming apparatus provided with a plurality of image forming portions of such a cleanerless type as described above along the movement direction of the transfer material suffers from the following problem.
In the developing device, not only the untransferred toner produced in each image forming portion, but also a toner (re-transferred toner) resulting from the re-transfer of part of a toner image formed by an upstream image forming portion with respect to the movement direction of the transfer material is collected. The re-transfer is the phenomenon that part of the toner image transferred to the transfer material in the upstream image forming portion with respect to the movement direction of the transfer material adheres onto the photosensitive member of a downstream image forming portion in the transferring portion of the downstream image forming portion. In the more downstream image forming portion with respect to the movement direction of the transfer material, the greater becomes the number of upstream image forming portions. Therefore, the more downstream image forming portion is greater in the amount of re-transferred toner, and becomes greater also in the amount of toner collected by the developing device.
Here, the untransferred toner and the re-transferred toner are toners, which could not be borne on the transfer material even if a transfer electric field was applied in the transferring portion of the image forming portion. Therefore, it is often the case that the charging charges of the untransferred toner and the re-transferred toner have a polarity opposite to a regular charging polarity or do not have a polarity. Also, it is often the case that the untransferred toner and the re-transferred toner are different-shaped toners as their toner shape or differ in their toner particle diameter from the mean particle diameter. Further, the re-transferred toner is part of a toner image comprising a toner of a discrete color formed in the upstream image forming portion and therefore, sometimes differs also in the nature of the toner.
The untransferred toner and the re-transferred toner, as described above, are returned to a proper charging amount by the charge assisting member provided downstream of the transferring portion with respect to the rotation direction of the photosensitive member and are collected by the developing device. However, the more downstream image forming portion is greater in the amount of re-transferred toner and therefore, the charge assisting member is liable to be contaminated by the toner and an extraneous additive or the like accumulating thereon. Therefore, the more downstream image forming portion becomes more insufficient in the control of the charging amounts of the untransferred toner and the re-transferred toner.
Thereby, it becomes liable to happen that the toner having passed the charge assisting member is not collected by the developing device or the developer in the developing device is increased in the amount of toner differing from the regular charging polarity. Therefore, a faulty image such as “fog” in which the toner adheres to a white background portion becomes liable to occur.
Also, the re-transferred toner is part of a toner image comprising a toner of a discrete color formed in the upstream image forming portion. Therefore, the mixing of colors occurs in the developing device into which the re-transferred toner has been collected, and it sometimes becomes impossible to reproduce an image of a proper color.